Imaging systems capable of detecting radiation quanta of different energy are typically used to acquire anatomical as well as functional information of an imaging subject. For example, SPECT enables the evaluation of disease processes based on functional and metabolic information of organs and cells. By integrating X-ray computed tomography into SPECT anatomical information may be combined with functional and metabolic information. SPECT/CT scanners have proven to be valuable in oncology and are recently attracting attention for novel applications in image guided therapy.
A radiation detector for combined detection of X-rays and gamma rays has been described in U.S. Pat. No. 6,448,559 B1. The detector has a multi-layered structure with several layers arranged one behind another. The detector comprises a front scintillator layer for detecting X-rays and a rear scintillator layer for detecting gamma rays disposed behind the front scintillator layer. The multi-layered structure further includes a photosensor layer which is arranged at a back side of the rear scintillator layer opposite the front scintillator layer so as to detect scintillation photons generated in both the front and the rear scintillator layers. X-rays, due to their comparably low energy, are typically absorbed by the front scintillator layer which is arranged facing the X-ray source.
Scintillation light generated by the absorption of an X-ray in the front scintillator layer is passing through the rear scintillator layer before being detected by the photosensor. Thus, scintillation light emanating from the front scintillator layer is spread over a larger area thereby reducing spatial resolution of the X-ray CT image, e.g. by reduction of the modulation transfer function (MTF).
Document EP2180342A1 discloses an X-ray line sensor in which a scintillator layer that absorbs X-rays in a low-energy range and emits light, and a scintillator layer that absorbs X-rays in a high-energy range and emits light are brought in contact with each other.
Document US2008/011960A1 discloses a radiographic imaging apparatus for taking X-ray images of an object. In various two-panel radiographic imaging apparatus configurations, a front panel and back panel have substrates, arrays of signal sensing elements and readout devices, and passivation layers. The front and back panels have scintillating phosphor layers responsive to X-rays passing through an object produce light which illuminates the signal sensing elements to provide signals representing X-ray images.
Document US2002/070365 discloses a system for spectroscopic imaging of bodily tissue in which a scintillation screen and a charged coupled device (CCD) are used to image selected tissue. An x-ray source generates x-rays which pass through a region of a subject's body, forming an x-ray image which reaches the scintillation screen.
Document US2013/126743 discloses a radiation detector that includes a scintillator layer, a first photoelectric conversion layer, a second photoelectric conversion layer, and one board or two boards. The first photoelectric conversion layer absorbs at least light of a first wavelength and converts the light to charges. The second photoelectric conversion layer absorbs more light of a second wavelength than of light of the first wavelength and converts the light to charges.